This invention relates to an echo removing apparatus and, more particularly, to an echo removing apparatus for reducing the echo caused by the sound generated returning from a speaker to a microphone of a small-sized communication terminal, such as a portable telephone.
In keeping up with reduction in size of the sound generated communication terminal, such as a portable telephone, the effect as of the echo produced by the sound generated returning from the sound generated-receiving speaker to the sound generated-sending microphone becomes hardly negligible. For removing the echo caused by the sound generated returning round on the transmitter/receiver, an echo removing apparatus or an echo canceler shown for example in FIG. 1 is employed.
Referring to FIG. 1, a terminal 11 receives a speaker output signal x(k) transmitted from a communication partner to a speaker 12, where k denotes a sample number or a time position of discrete signals. A microphone input signal y(k), collected by a microphone 13 and thereby converted into an electrical signal, is supplied along with a pseudo echo signal supplied from a filter circuit 15 to a subtractor 14. The subtractor subtracts the pseudo echo signal supplied from the microphone input signal to form a resultant echo-reduced signal or a residual echo signal e(k) which is supplied to an input terminal 16. In a portable telephone, the speaker 12 and the microphone 13 are usually arranged close to each other as a telephone handset.
For the adaptive filter 15, a so-called finite response (FIR) filter is employed. The filter coefficients or tap coefficients are set for minimizing the error signal(k). The adaptive filter 15 filters the input signal, that is the speaker output signal x(k), for estimating the echo signal for generating the pseudo echo signal. This pseudo echo signal is provided to the subtractor 14 where the pseudo echo signal is subtracted from the microphone input signal y(k) to derive the residual echo signal or error signal e(k).
That is, if the input signal supplied from the terminal 11, that is the speaker output signal x(k), is the tap input to the N-tap FIR filter, operating as the adaptive filter 15, and the tap coefficients of the adaptive filter 15 are bk(i), where i=0, 1, . , N−1, the pseudo echo signal outputted by the adaptive filter 15 is given by
                                          y            ^                    ⁡                      (            k            )                          =                              ∑                          i              =              0                                      N              -              1                                ⁢                                                    b                k                            ⁡                              (                i                )                                      ⁢                          x              ⁡                              (                                  k                  -                  1                                )                                                                        (        1        )            
The subtractor 14 subtracts the pseudo echo signal of the equation (1) from the microphone input signal y(k) to derive the residual echo signal or error signal e(k) by:e(k)=y(k)−ŷ(k)  (2)
The tap coefficient {bk(i)} of the adaptive filter 15, where i=1, 2, . . . , N−1, is updated, by a suitable algorithm, such as a least mean square (LMS) algorithm, learning identification method or a recursive least square (RLS) algorithm, for minimizing the time average of the power of the error signal e(k) of the equation (2), that is,E[∥e(k)∥2]where E[ ] is an expected value or a mean value of the value within the brackets [ ] and ∥e(k)∥2 is the square sum of e(k). The tap coefficients of N taps of the adaptive filter 15 are equivalent to estimated values of the echo characteristics between the speaker 12 and the microphone 13.
Meanwhile, if desired to reduce the number of tapes N to the smallest possible value for simplifying the structure, the frequency characteristics of the residual echo are left in the low frequency range, even though the echo characteristics can be estimated partially with high accuracy by the adaptive filter 15, thus raising difficulties in effectively canceling the echo from sound generated input signals containing strong low-frequency components.
FIG. 2A shows the impulse response of the echo signals in association with the respective taps of the FIR filter. FIG. 2B, on the other hand, shows the impulse response of the residual echo signal obtained on subtracting the pseudo echo signal estimated by 20-tap FIR adaptive filter. It is seen from FIGS. 2A and 2B that the impulse response of the residual echo signal corresponding to 20 taps has been removed.
FIG. 3 shows, in association with FIGS. 2A and 2B, a spectral curve a of the echo signal and a spectral curve b of the residual echo signal produced on subtracting the estimated echo signal supplied from the 20-tap FIR adaptive filter.
Thus, even if it may appear that the echo characteristics can be substantially estimated by the 20-tap FIR adaptive filter, the frequency characteristics of the residual echo characteristics (curve b) are left in a region from 500 Hz to 1 kHz where the energy of sound generated signal would be concentrated to a larger extent.